I would like to set up mail address of "mail@domain.com".

However, the user "mail" cannot be created because it says "mail" is reserved.

And I set up another user name with e-mail alias "mail".

Now when I send mail to mail@domain.com, I receive the error message:

Deferred: mail.domain.com.: No route to host......

Does anyone have any idea how to configure/fix this situation??